This invention relates to the curing of an epoxy adhesive using infrared and/or near-infrared energy (hereinafter collectively referred to as “IR energy”). In particular, the invention relates to encapsulating an epoxy adhesive component in a releasable microcapsule which disintegrates upon being exposed to IR energy thereby releasing the encapsulated epoxy adhesive component and resulting in the curing of the epoxy adhesive.
Adhesives are used widely throughout numerous industries and are especially useful in the manufacture of electronic devices. For example, adhesives are widely used for attaching semiconductor chips with projecting electrodes to wiring substrates. It is desirable to apply and activate adhesives cleanly, safely and rapidly. Nearly all electronic device adhesives utilize some form of energy to activate the adhesives (e.g., thermal or ultraviolet).
The laser is an important source of IR energy in electronics manufacturing. For example, solid-state diode lasers are widely used due to their simplicity, low cost and high efficiency to selectively heat solders. One such laser is a diode type laser, model DLS, available from Speedline Technologies/Electrovert of Camdenton, Mo. which emits at a wavelengths between about 802 to 808 nm. Lasers have also been used to bond thermoplastic adhesives together and to other materials such as glass. Thermoplastic materials, however, absorb little or no IR energy so an IR absorber (e.g., carbon and/or IR absorbing dyes) must be added to the thermoplastic adhesives.
Thermoset adhesives (e.g., epoxies) are generally considered to be superior to thermoplastic adhesives because thermoset adhesives can be applied as a liquid that is easy to print or dispense with a needle onto a substrate. Further, thermoset adhesives are more heat resistant than thermoplastic adhesives which soften and expand as temperatures increase. Presently, thermoset adhesives are usually cured by heating an assembly being joined to a temperature of at least about 150° C. for a duration of about 5 minutes to about 6 hours. Microwave energy has been used to decrease the duration of the thermal curing operation. This approach, however, has disadvantages that prevent its widespread use. For example, microwave generators present radiation hazards and special safety features must be maintained. They also cause radio interference and as a result must be licensed by governmental authorities. The equipment is costly, and some products can be damaged by the powerful radiation. For example, some conducting materials have a resonance frequency near that of the microwave energy which can damage the product and cause fires. Solid-state devices are particularly susceptible to damage caused by electrical charge and heat generated by absorbing the radiation. Even newer microwave ovens which rapidly vary their frequencies can cause damage, in fact, they must be adjusted for each type of part being treated.
Ultraviolet-activated adhesives have also been used in the electronic device manufacturing industry. Ultraviolet-activated adhesives require the use of transparent substrates, thus, their use is limited. For example, they are typically only used to bond glass to ceramic packaging. UV-activated adhesives are also not as versatile as thermoset adhesives because, for example, the polymerization reactions typically generate low molecular weight by products that have inferior properties compared to thermoset reactions and their resulting products. Additionally, the final properties of the cured adhesive can vary widely because the polymerization reaction depends greatly on relatively minor variations in machine parameters and coating or adhesive thickness. Further, many of the radiation polymerization reactions are inhibited by oxygen and/or moisture from the atmosphere and it may be necessary to run them in inert gas atmospheres which increases production costs. One more problem is that depth of UV radiation penetration is limited to only a few thousandths of an inch. UV curing is also a highly thermal process which can damage a treated substrate and/or component (about 50% or higher of UV lamp output is waste heat).
In view of their superior properties, thermosetting coatings and adhesives are widely used and would be desirable to quickly cure them with IR energy. Although IR absorbing materials have been added to thermoset adhesive, this is not an optimal approach because the IR absorbing dyes produce anisotropic heating, specifically, most of the heating occurs only at the surface. Also, current methods of using IR energy to trigger curing fail to address the latency problem (i.e., short pot life at ambient temperatures) of thermoset adhesive because the IR energy is simply being used as a heat source to raise they temperature of the adhesive and thereby increase the polymerization reaction. Because reaction rates typically double with every 10° C. rise in temperature, thermoset materials must either be catalyzed just prior to use, or stored in a freezer and used before they polymerize, or partially polymerize to a point where the viscosity is too high for normal application.
To date, therefore, methods of curing thermoset adhesive with IR energy have not been satisfactory. As such, a need exists for a simple, cost-effective approach for curing thermoset adhesives with IR energy.